The technique employed in this particular type of printing technology (Thermal Ink-Jet, generally abbreviated TIJ) is well known and is described in numerous publications and Patents; in short, the TIJ technology uses thermal energy to expel droplets of ink towards the paper.
The energy is supplied by a resistor which, located at the bottom of an expulsion chamber, brings the ink situated above it to boiling point; the gaseous bubble thus formed pushes the ink above until it is expelled through a nozzle on to the paper. During the expulsion process, a meniscus of liquid is formed in the region of each nozzle which, by breaking and reforming again, regulates proper formation and expulsion of the droplet of ink.
The ink is contained in a ink reservoir, including for example a porous medium, from which it flows in controlled manner towards the expulsion chamber; the reservoir may be an integral part of the head or may be a separate object, connecting to the printhead, as in the case of the refillable heads.
In the TIJ technology specific, specially formulated inks are used to offer a plurality of precise requirements such as, for example, the following:
1. chemical/physical properties suitable for the technology in question, i.e. pH between 7.5 and 8.5, viscosity varying from 1 to 5 mPa.times.second @25.degree. C., surface tension ranging from 25 to 50 dyne/cm @25.degree. C.,
2. complete thermal stability so as not to form insoluble residues on the resistor element during the heating process,
3. capable of producing high quality images on the widest range of papers and, in general, on various print media,
4. good properties of interaction (fixing) with the print medium and therefore good performance in terms of resistance to water and light,
5. drying times allowing manipulation of the sheet immediately after printing,
6. perfect operation of the printhead all throughout the product's life,
7. stability in time from the chemical/physical and microbiological viewpoints,
8. completely compatible with the other materials comprising the printhead,
9. absolutely safe to use, in consideration of both the user and the surrounding environment.
Ink compositions are known in the technique that possess the above properties to varying degrees, such as, for example, those described in the U.S. Pat. Nos. 5,098,476; 5,106,416; 5,156,675; 5,165,968 and 5,188,664.
However, as all these inks are formulated with an aqueous base and dyes that are soluble in the base, the drying speed and water resistance are not always completely satisfactory even when the print medium (paper) used is of good quality; then when coming to print on types of paper such as, for example, "recycled" papers, the print quality that is obtained from the known inks is not satisfactory.
With particular regard to drying speed, it is known that the inks typically used in this type of technology are characterized by a water content of about 90% which, though print quality and optical density are good, inevitably leads to lengthy drying times.
Drying time, in fact, is essentially linked to the speed of evaporation and penetration in the print medium for like quantities of ink; the solution in which the amount of ink expelled is reduced and smaller droplets are used means that drying time decreases but at the same time leads to undesirable reductions of optical density and, more generally, of contrast on the medium.
However, different formulations are known in which evaporation speed is increased, for example by adding to the ink large amounts of volatile substances, typically short-chain alcohols, but this solution also has the drawback that optical density is greatly diminished.
Also, addition of these substances often results in decreased decap time; decap time is the amount of time elapsing before the ink, on exposure to air in the region of the printhead nozzles, solidifies and can no longer be expelled or is not expelled properly. The above-mentioned substances, which greatly increase vapor pressure of the ink, have a negative impact on this factor, which is extremely important for the operation of ink-jet printheads.
Other formulations are also known that promote an increase in the amount of penetration, obtained by the addition of surface-active agents; however, in this case, as well as a sharp decrease in optical density, print quality is also often poorer in terms of definition, of feathering (i.e. the tendency of the ink to spread through the paper fibers) and pass-through (the phenomenon wherein the ink passes right through the sheet of paper so that the printed characters are also visible from the rear side of the sheet). Surface-active agents, by lowering the surface tension of the ink, can also result in deterioration of the meniscus formed in the expulsion phase.
As regards water resistance, using dyes that are soluble in water, though this on the one hand guarantee stability of the ink over long periods without any precipitations forming, on the other hand entails poor resistance of the dyes in question to water.
As a solution to this problem, use was proposed of inks containing not dyes soluble in water but pigments which, as is known, are absolutely insoluble in water. These pigmented inks, however, have two drawbacks: stability over time of the pigment dispersion and drying time.
Stable dispersions, in fact, require use of an amount of co-solvent which inevitably greatly impairs drying time. Moreover, the addition of surface-active agents, as well as being critical for the reasons already examined regarding water-soluble dyes, is also a factor having a destabilizing effect on pigment dispersions (as illustrated, for example, in U.S. Pat. No. 5,169,436).
A final critical aspect regarding the inks known today, as mentioned earlier, derives from the possibility of using a wide range of print papers, for example recycled papers, produced not from virgin raw materials but from salvage materials.
This feature has recently become particularly important. Whereas the ink-jet printer market up to now has been geared towards use of plain papers, with which the current inks provide average-to-good quality printing on a wide range of papers marketed as "Xerographic" paper (i.e. originally intended for the market of photocopiers using the Xerographic technology), the recent spread of these recycled papers together with the increased use of laser printers (on which, as is known, use of recycled papers presents no particular difficulties) has created new expectations among users of ink-jet printers as well.
The problem of using recycled papers has not yet been fully solved from the viewpoint of formulating valid inks for ink-jet printers, for it is true that the current inks produce an unacceptable level of print quality on this type medium, mainly because of interaction with the fibers of the paper which leads to extensive "feathering" and a conspicuous loss of definition.
The current situation can be best be illustrated by way of two examples of ink compositions, identified as "Standard 1" and "Standard 2", which represent the state of the art, and their principal properties.
______________________________________ Ink composition "Standard 1" ______________________________________ Diethylene glycol 5.0% by weight Food Black 2 3.0% by weight Surfynol 104 E 0.05% by weight Phosphate buffer 2.5% by weight Preventol D6 0.1% by weight Deionized water 89.35% by weight ______________________________________
This example has the following properties (significance of these properties and how they are measured will be described in detail later):
______________________________________ pH 8 surface tension 44.5 dyne/cm viscosity 1.3 mPa .times. s drying time 80 s water resistance DL* = 27.0 "feathering" on Xerographic paper 5 on recycled paper 1 definition good print quality on recycled paper very poor. Ink composition "Standard 2" Diethylene glycol 5.0% by weight ProJet Fast Black 2 0.5% by weight Bayscript Schwarz N01 2.0% by weight Surfynol 104 E 0.05% by weight Phosphate buffer 2.5% by weight Preventol D6 0.1% by weight Deionized water 89.85% by weight ______________________________________
It has the following properties:
______________________________________ pH 8 surface tension 43.0 dyne/cm viscosity 1.3 mPa .times. s drying time 70 s water resistance DL* = 3.8 "feathering" on Xerographic paper 5 on recycled paper 1 definition good print quality on recycled paper very poor. ______________________________________
From the foregoing examples, it is clear, as previously stated, that drying time, water resistance and usability of recycled papers represent a problem with the current inks.